According to a study made by the inventors of the present invention, the following techniques are known as the conventional application technique of the backup data.
For example, a backup system of a database using two computer systems has been widely used. The backup of the database means creating the same data as that of one computer system and storing the created date in the other computer system. The computer system used as a backup source is called as a primary system, and the computer system used as a backup destination is called as a backup system. Further, the data stored in the backup system is called as a backup data. The backup of the database by a duplex system is described in the document, for example, CHRISTOS A. POLYZOIS, HECTOR GARCIA-MOLINA, “Evaluation of Remote Backup Algorithms for Transaction-Processing Systems”, ACM Transactions on Database Systems, Vol. 19, No. 3, September 1994, Pages 423-449.
As a backup method of the database by the duplex system, a log transfer method is known. The log transfer method is a method of transferring a data update record of the primary system to the backup system and creating the backup date in the backup system. In the log transfer method, the data in the primary system is made identical to the backup data in the backup system at a stage of starting the backup. After starting the backup, the data update record of the primary system is transferred to the backup system. In the backup system, the backup data is updated on the basis of the transferred data update record to create the latest backup data. The log transfer method is described in the above-identified document.
In general, the computer system includes an external storage device, and the data of the computer system is stored in the external storage device. The external storage device is a device for storing the date of the computer system, and is exemplified by magnetic storage devices such as a hard disk and a disk array.
Some external storage devices have a copy function to copy the data between two devices. For example, the copy function is described in Japanese Patent Application Laid-Open No. 2004-78746. In this copy function, the data is transferred via a network so that the data in one external storage device is copied to the other external storage device. In the case of using this copy function, when the data is added to the external storage device used as a copy source, the same data is added also to the external storage device used as a copy destination.
The backup in accordance with the log transfer method is executed by using the copy function of the external storage device. A mechanism thereof is as follows. At a time of starting the backup, the data in the primary system is made identical to the backup data in the backup system. The data update record added to the external storage device of the primary system is copied to the external storage device in the backup system by using the copy function for the data update record. In the backup system, the latest backup data is created on the basis of the copied data update record.
Further, the technique of duplexing the backup data in the backup system has been prevailing. In the case of duplexing the backup data, two areas for storing the backup data are prepared in the external storage device of the backup system. The backup data is created in one storage area in accordance with the log transfer method, and the backup data is copied to the other storage region.
A method of copying the storage area of the external storage device is described in, for example, Japanese Patent Application Laid-Open No. 2003-345523. This copy function is a function to copy the data in two designated storage areas, and it is possible to duplex the backup data by using this function.
In this case, as a result of the study the inventors of the present invention for the conventional application technique of the backup data as mentioned above, the following matters have been found out.
For example, there has been conventionally a problem that the backup data of the backup system includes an unavailable data. This is caused by the fact that the data in which an uncommitted transaction is active is present in the backup data of the backup system. The transaction means a series of data operation, and the data operation becomes effective at the time when the transaction is committed. The data in which the uncommitted transaction is active is an indeterminate data, and the indeterminate data cannot be utilized.
For example, the case where the data of the primary system is updated by a certain transaction will be described. When the transaction updates the data of the primary system, the data update record is transferred to the backup system, and the data in the backup system is also updated. However, the data operation is not effective until the transaction is committed. For example, there is a case where a rollback is executed before the transaction is committed, and the data is returned. Accordingly, the backup data includes the unavailable indeterminate data from the time when the data is updated to the time when the transaction is committed.